The present invention relates to longitudinal magnetic recording media, such as magnetic drums, magnetic tapes, magnetic disks, and magnetic cards, and magnetic recording devices. In particular, the present invention relates to a longitudinal magnetic recording medium suitably used for ultrahigh-density recording of 70 Gigabits or more per square inch.
Owing to popularization of the Internet vis-à-vis the World Wide Web (WWW), the demand for magnetic disk devices is increasing along with the increase in the number of personal computers on the market. Although it is possible to access the Internet from mobile terminals as well as from personal computers, it is necessary to incorporate a magnetic disk device into a mobile terminal to pursue further convenience of the mobile terminals. It is expected that the demand for mobile terminals provided with magnetic disk devices will increase in the future. Further, because of the impending digitalization of TV broadcasting, a full-scale use of the magnetic disk device as a recording device has already started. The applicable field of the magnetic disk device is thus extending, and end users are requesting magnetic disk devices of reduced size and significantly increased capacity.
A reduction in noise of the longitudinal magnetic recording medium is efficient means for achieving the high recording density of the magnetic disk device. In order to realize the reduction in noise, it is very important to refine magnetic crystal particles. However, when the volume of a crystal particle becomes extremely small, due to refinement of the magnetic crystal particles, the magnetic crystal particles are susceptible to thermal energy even at an ordinary temperature, so that recorded magnetization decays over time. This phenomenon is generally called “thermal fluctuation.” The index used to indicate resistance to thermal fluctuation is Ku·v/kT. This index indicates how many times the product of anisotropic energy Ku of a magnetic body and a volume v of a magnetic crystal particle is greater than room temperature energy kT. The larger the value of this index, the greater the thermal stability. There are various methods for measuring Ku·v/kT. In the method employed in embodiments of the present invention, it is obtained in accordance with the time dependence of remnant coercive force (Hr).
It is impossible to satisfy the function of a recording device if the recorded information is lost; therefore, a medium having a structure described in Japanese Patent Laid-open No. 2001-56924 has been proposed as a countermeasure.
This medium disclosed in Japanese Patent Laid-open No. 2001-56924 is generally known as an AFC (antiferromagnetically coupled) medium, which comprises at least two magnetic layers with an intermediate layer formed from Ru or the like interposed therebetween. In a state where no magnetic field is externally applied, an upper magnetic layer and a lower magnetic layer are antiferromagnetically coupled by Ru. In short, magnetizations of the upper layer and the lower layer are antiparallel to each other. Such specific magnetic coupling is realized by optimizing a thickness of Ru. Further, since the medium noise is reduced with a reduction in antimagnetic field encountered in a magnetization transition region, it is possible to reduce the medium noise simply by maintaining the product (Br·t) of a remnant magnetic flux density and a magnetic film thickness at a small value. Since the upper magnetic layer and the lower magnetic layer are antiferromagnetically coupled in the AFC medium, an effective value of Br·t is the difference between the upper and lower magnetic layers. Accordingly, since it is possible to reduce Br·t without extremely reducing the volume of a magnetic crystal particle via the AFC medium, the medium noise can be reduced with the thermal fluctuation resistance being secured.
At present, glass is often used as a substrate of the magnetic disk medium. When using a glass substrate, a seed layer to be formed thereon has an important function in controlling the crystal orientation of a magnetic layer. In an ordinary longitudinal magnetic recording medium, Co having an hcp crystal structure is often used as a main component of the magnetic material, and its easy axis of magnetization is in the direction of the c-axis. In order to simultaneously secure thermal fluctuation resistance and improve recording density in the longitudinal recording medium, it is particularly important to positively orient the easy axis of magnetization in the film. PCT Publication No. WO00/60583 and Japanese Patent Laid-open No. 2001-14325 each propose TiTl as a seed layer material (to be formed directly on the substrate) for positively longitudinally orienting the c-axis of a magnetic layer having an hcp structure.
The recording medium disclosed in PCT Publication No. WO00/60583 has a structure in which a TiAl layer is formed on a substrate. A magnetic film is formed directly on the TiAl layer or with an underlayer of Cr or with a Cr alloy disposed therebetween.
The recording medium disclosed in Japanese Patent Laid-open No. 2001-14325 has a substantially similar structure, but an example of forming the TiAl layer indirectly on the substrate is disclosed therein. For example, a recording medium structure of Cr-based underlayer/TiAl underlayer/Cr-X seed layer/NiP layer/substrate is disclosed in Example 1. Further, a structure of magnetic film/CoCr layer/Cr-based intermediate layer/TiAl underlayer/B2 structure seed layer/substrate is disclosed in Example 3.
Examples of two-layered seed layers wherein a RuAl seed layer is formed on each of TiAl, CrTa, and AlTa seed layers are disclosed in U.S. Patent Application Publication No. 2001/008136. The recording medium includes a CrTi layer formed as an underlayer of the RuAl seed layer.
A study was carried out to realize a recording medium having thermal fluctuation resistance and high recoding density by using the AFC structure and by strongly orienting the easy magnetization axis of a magnetic film . According to our study, in order to assure reliable performance of a magnetic disk device, particularly in order to prevent the recorded information from deteriorating due to the influence of thermal fluctuations, it is necessary to keep the value of Ku·v/kT at 70 or more. Our study confirmed that the orientation of the magnetic film improved to achieve a satisfactory thermal fluctuation property by using a two-layered seed layer, including the RuAl seed layer, which is disclosed in Patent Document 4. It was also confirmed that Ku·v/kT depends greatly on the first seed layer material and the thickness of the RuAl seed layer, and that the RuAl seed layer thickness must be 12 nm or more. However, since Ru is very expensive and should be used in a ratio of 1:1 to Al, the use of Ru entails a remarkable increase in cost of the medium. Further, in order to increase the thickness of the RuAl seed layer, input power must be increased for sputtering, thereby causing problems of increases in dust and defects.